The present invention relates to a master cylinder for supplying brake fluid to a brake apparatus of a vehicle.
Generally, a master cylinder of this type comprises: a cylinder that has a sleeve incorporated into a cylinder body and that is also provided with a discharge passage for and a supply passage for brake fluid; a cup-shaped piston (a plunger) which is slidably disposed in the cylinder to thereby form a pressure chamber between the piston and the cylinder, the pressure chamber supplying a fluid pressure to the discharge passage; and a piston seal which is provided between the cylinder and the piston and is capable of sealing a space between the supply passage and the pressure chamber. However, a cylinder formed by combining a cylinder body and a sleeve, such as the one described above, obviously requires more parts. Further, the radial size of the cylinder becomes larger, resulting in a larger overall size of the master cylinder. Therefore, it has been proposed to use a master cylinder in which a piston is directly inserted into a cylinder body without use of a cylinder sleeve (see, for example, U.S. Pat. No. 4,524,585).
In this master cylinder, a cylinder body includes a communication groove which opens into a circumferential groove and extends from the circumferential groove toward a closed end of the cylinder body, to thereby allow communication between a discharge passage and the circumferential groove. When brake fluid is filled into the cylinder body, a piston seal is opened due to a pressure difference, and the brake fluid is flowed into the discharge passage through a gap between the piston seal and a bottom surface of the circumferential groove, and the communication groove. However, in this master cylinder, the communication groove is located inward of the bottom surface of the circumferential groove, relative to a radial direction of the cylinder. Therefore, when brake fluid is filled, a negative pressure in the cylinder body acts on the piston seal. This causes an end portion of an outer circumferential surface of the piston seal to rise from the bottom surface of the circumferential groove and to adhere to a side surface. Under these circumstances, a flow of brake fluid into the communication groove through the gap between the outer circumferential surface of the piston seal and the bottom surface of the circumferential groove is obstructed by the piston seal adhering to the side surface. As a result, the supply of the brake fluid is restricted and a brake-fluid filling operation sometimes has to be repeated several times.